1. Field of the Invention
The present invention relates to a power supply apparatus that generates from an input voltage an output voltage that is permitted to vary within a predetermined permissible variation range.
2. Description of the Prior Art
Conventionally, in a power supply apparatus that generates from an input voltage Vi an output voltage Vo that is permitted to vary within a predetermined permissible variation range, the output voltage Vo is kept equal to a predetermined reference voltage Vref (a constant level that is not influenced by a increase or decrease in the output current Io) through the feedback control of the output voltage Vo (see FIG. 7A).
It is true that, with the power supply apparatus that performs the control described above, a load can be supplied with the output voltage Vo of the power supply apparatus as a voltage of which the variation is within the permissible variation range even when the input voltage Vi or the output current Io varies slightly.
However, in the power supply apparatus that performs the control described above, when the output current Io varies so abruptly as not to be coped with by the feedback loop of the output voltage Vo, a large variation occurs in the output voltage Vo, causing, in the worst case, the output voltage Vo to go out of the permitted variation range (see FIG. 7B). This is one problem with the conventional power supply apparatus. A solution to this problem has been becoming especially critical in recent years as semiconductor chips (such as CPUs) used as a load for such a power supply apparatus consume increasingly large currents and operates at increasingly high speeds and thus require increasingly stable output voltages Vo even in the event of an abrupt variation in the load. Moreover, in the power supply apparatus configured as described above, the feedback control is so performed that the output voltage Vo is kept equal to the reference voltage Vref that is not influenced by an increase or decrease in the output current Io. This causes the power consumption by the load to increase as the output current Io increases. This is another problem with the conventional power supply apparatus.
Various techniques have conventionally been disclosed and proposed to give a solution to the problems mentioned above (for example, see Japanese Patent Application Laid-Open No. 2002-186254). However, all of those techniques attempt to solve the problems by increasing the response of the feedback loop, and are thus basically no different from the conventional configuration described above in that they perform feedback control so that the output voltage Vo is kept at a constant level that is not influenced by an increase or decrease in the output current Io. As long as such a configuration is used, the transient characteristic of the output voltage Vo cannot be improved beyond a certain limit. That is, in the event of an abrupt variation in the output current Io, there still is a risk, in the worst case, of the output voltage Vo going out of the permitted variation range. Moreover, as the output current Io increases, the power consumption by the load still tends to increase.